There are many different types of hinges for plastic box covers, but in cases where assembly is needed, it sometimes takes a difficult procedure to align the male and female parts for snapping the cover hinge portion to the box hinge portion; and in other cases when the box is open, it may open too far or sag to a position more than 180.degree. with respect to the box. In some cases this is not desirable as for instance in a first aid plastic kit; and it is the object of the present invention to provide a snap-in type plastic hinge construction wherein the box can only be assembled in one way and in which at the same time the elements that go to prevent misassembly of the parts also provide against opening of the box past 180.degree..